1. Field of the Invention
This invention relates to an optical disk unit which reads information from a recording medium, such as an optical disk and a opto-magnetic disk, or reads and writes information relative to such a recording medium.
2. Description of Related Art
Recently, optical disk units have become compact and thin in size, and are of a high performance design so that they can be mounted on a smallsize information processing device.
A conventional optical disk unit will now be described. FIG. 20 is a view showing important portions of the conventional optical disk unit, and FIG. 21 is a view showing a pickup head of the conventional optical disk unit. As shown in FIG. 20, the optical disk unit comprises a spindle motor 2 for rotating an optical disk (recording medium) 1, a fixed optical portion 3 and a movable optical portion 4 which reproduce data stored in the optical disk 1, or record and reproduce such data, and a feed portion 5 for moving the movable optical portion 4 in a direction radially of the optical disk 1. A laser beam emitting means for applying a laser beam to the optical disk 1, a photo detector means for detecting the light or beam reflected from the optical disk 1, and other optical parts are mounted on the fixed optical portion 3. Optical parts which direct the laser beam, applied from the fixed optical portion 3 to the optical disk 1, and also condense or converge the laser beam, a focusing drive means, and a tracking drive means are mounted on the movable optical portion 4. In FIG. 21, an objective lens 6 for converging the laser beam, emitted from the fixed optical portion 3, onto the optical disk 1, a bobbin 7 supporting an objective lens 6, focusing coils 8 and tracking coils 9 are mounted on the bobbin 7. The bobbin 7 is resiliently supported by suspension springs 10 for movement in a focusing direction (direction of an arrow A) and a tracking direction (direction of an arrow B) relative to a head base 11. Opposite ends of each suspension spring are fixedly secured to the head base 11 and the bobbin 7, respectively. A pair of magnets 12 are fixedly mounted on the head base 11 in opposed relation to the focusing coils 8 and the tracking coils 9. The pickup head shown in FIG. 21 is mounted on a carriage base 13 shown in FIG. 20, and guide rollers 14 mounted on the carriage base 13 are held in engagement with two guide shafts 15, so that the pickup head is slidable, together with the carriage base 13, in a direction radially of the optical disk 1. Feed magnets 16 are fixedly mounted respectively on back yokes 17 of a ferromagnetic material. An opposed yoke 18 made of a ferromagnetic material is provided in opposed relation to a respective one of the feed magnets 16. The feed magnet 16, the back yoke 17 and the opposed yoke 18 jointly constitute a magnetic circuit of the feed portion 5. A pair of feed coils 19 are mounted on the carriage base 13, and the opposed yokes 18 extend through these feed coils 19, respectively.
The operation of the optical disk unit of the above construction will now be described. A laser beam, emitted from the laser beam emitting means mounted on the fixed optical portion 3, is directed to the objective lens 6 by an upwardly-directing mirror and other associated parts mounted on the head base 11 of the movable optical portion 4, and is converged onto the optical disk by the objective lens 6. On the other hand, reflection light from the optical disk 1 is fed via the objective lens 6 and the upwardly-directing mirror and other associated part to the photo detector means mounted on the fixed optical portion 3. Using a signal outputted from this photo detector means, data stored in the optical disk 1 is reproduced. In order to satisfactorily reproduce the data stored in the optical disk 1, the objective lens 6 needs to be moved in the focusing direction (direction of the arrow A) and the tracking direction (direction of the arrow B) so as to accurately converge the laser beam, emitted from the laser beam emitting means mounted on the fixed optical portion 3, onto the optical disk 1. With respect to the movement of the objective lens 6 in the focusing direction, the focusing drive is effected by an electromagnetic effect, achieved by the focusing coils 8 and the magnets 12, against the bias of the suspension springs 10. With respect to the movement in the tracking direction, the tracking drive is similarly effected by an electromagnetic effect achieved by the tracking coils 9 and the magnets 12. The carriage base 13 is moved along the guide shafts 15 by an electromagnetic effect achieved by the feed coils 19 and the feed magnets 16. By thus driving the carriage base 13, a feed drive is effected such that the objective lens 6 moves over an entire data area of the optical disk 1.
By the above focusing drive, tracking drive and feed drive, the laser beam, emitted from the fixed optical portion 3, is accurately converged onto a required position of the optical disk 1 through the objective lens 6.
In the above conventional construction, however, the carriage base 13 is of such a construction that the opposite sides thereof are disposed close to the pair of guide shafts 15, respectively, and therefore the carriage base 13 is large in size and heavy in weight. As a result, the carriage base 13 can not be driven at high speed in the tracking direction. Further, two drive portions are required, that is, the tracking drive portion constituted by the tracking coils 9 and the magnets 12 for moving only the bobbin 7, having the objective 6 mounted thereon, in the tracking direction and the feed drive portion constituted by the feed coils 19 and the feed magnets 16 for moving the entire movable optical portion 4, including the carriage base 13, in the tracking direction. Thus, the number of component parts increases, and therefore it is difficult to provide a small-size design and to reduce the cost. Further, since the fixed optical portion 3 is separate from the movable optical portion 4, it has been necessary to achieve such a positional accuracy that the laser beam, emitted from the laser beam emitting means, can be directed to the objective lens 6. Thus, high precision of the parts and means for making adjustments during assembly are required. Furthermore, in the magnetic circuit constituted by the magnets 12, the focusing coil 8 and the tracking coil 9 are disposed in overlapping relation to each other, and therefore a gap in the magnetic circuit is increased by an amount corresponding to the thickness of this overlap, which has resulted in decreasing the efficiency with which the actuator is driven.